1. Field of the Invention
The present invention relates to a charging apparatus. More specifically, the present invention relates to a charging apparatus of a rechargeable battery using an alkali type electrolyte such as a sealed type Ni-Cd battery.
2. Description of the Prior Art
Of late, a motor driven machines having a built-in rechargeable batteries in lieu of a commercial power supplies have been put into practical use. Generally, batteries employing an alkali type electrolytes such as a Ni-Cd have been used as a voltage sources of such motor driven machines. FIGS. 1 to 3 are graphs showing various characteristics in conjunction with charging of such Ni-Cd batteries. More specifically, FIG. 1 is a graph showing a change of a battery voltage and a change of a battery temperature while charging a battery. Referring to FIG. 1, the curve I indicates the battery voltage and the curve II indicates the battery temperature. It is pointed out that FIG. 1 shows the characteristic on the assumption of the charging conditions that both the ambient temperature (Tamb) and the charging current (Ic) are constant. As seen from the curve I in FIG. 1, when the charge quantity with respect to the battery capacity reaches 100%, this means charge completion. The battery voltage when the charge is completed, i.e. the voltage at charge completion is shown as Va and an increase of the battery temperature at the charge completion is shown as Ta. The curve III in FIG. 2 shows a change of the voltage at charge completion (Va) with respect to the ambient temperature (Tamb). The curve IV in FIG. 3 shows an increase of the battery temperature (Ta) at charge completion with respect to the ambient temperature (Tamb). Thus, generally a Ni-Cd battery has a non-linear characteristic of the voltage at charge completion (Va) with respect to the ambient temperature (Tamb). Conversely, an increase of the battery temperature (Ta) at completion exhibits approximately a linear characteristic with respect to the ambient temperature (Tamb).
FIG. 4 is a schematic diagram of one example of a conventional charging apparatus of interest to the present invention. The FIG. 4 apparatus is adapted to control a charging operation as a function of the above described battery voltage. Such charging circuit as similar to FIG. 4 is disclosed as FIG. 2 in Japanese Patent Laying Open Gazette No. 139033/1979, laid open Oct. 29, 1979 for public inspection, which corresponds to United States application, Ser. No. 891,305, filed Mar. 29, 1978, now U.S. Pat. No. 4,240,022. More specifically, an alternating current voltage such as a commercial power supply, received through a plug 2, is transformed by a transformer 3 and is then applied to a full-wave rectifying circuit 4. The full-wave rectifying circuit 4 is connected to a series connection of a battery 1 to be charged, a thermal switch 5 and a thyristor 6. The thermal switch 5 may be of a type operable as a reed switch utilizing a transition at a Curie point such as that of a temperature sensitive ferrite. The thermal switch 5 has been set such that if and when the temperature of the battery 1 reaches a predetermined value, say 47.degree. C., the thermal switch 5 becomes operable, so that the same may be rendered non-conductive.
In operation, at the beginning of a charging operation, the temperature of the battery 1 is low and accordingly the thermal switch 5 has been rendered conductive. When the plug 2 is connected to the commercial alternating current voltage source, a current flows to a capacitor 9 through the rectifying circuit 4, the battery 1 and the diode 7. The capacitor 9 provides a pulse voltage to the gate electrode of the thyristor 6, so that the thyristor 6 is turned on and the battery 1 starts being charged. As the charging time period lapses, the battery voltage increases. When the charged quantity reaches approximately 80% of the battery capacity, the battery 1 gives rise to an increase of internal resistance due to gas pressure generated in the battery. Thereafter the temperature of the battery 1 also increases. When the temperature of the battery 1 reaches a predetermined value, the thermal switch 5 is rendered non-conductive. Accordingly, a trickle charging is started in the battery 1 through the path of the diode 7, the resistor 8, the capacitor 9 and the resistor 10. A charging current on the occasion of the trickle current charging is approximately 1/10C to 1/5C, where C is a battery rate and represents a charging current required for charging the battery in an hour. A zener diode 11 is rendered conductive due to a charging voltage across the capacitor 9 included in the trickle charging path. When the zener diode 11 is rendered conductive, the gate electrode of the thyristor 6 decreases to a voltage too small to turn on the thyristor 6 in cooperation with the resistors 12 and 10. Accordingly, even if the thermal switch 5 is rendered conductive again by a decrease of the temperature of the battery 1, the thyristor 6 will not be turned on again and the battery 1 will not be charged. Thus, with the FIG. 4 apparatus, the temperature of the battery 1 is detected and a charging current is switched from a rapid charging operation to a trickle current charging operation depending on the temperature thereof. However, the operating temperature of the thermal switch of such conventional apparatus is constant irrespective of a change of the ambient temperature. Therefore, the problems to be described in the following are involved.
FIGS. 5A and 5B are graphs for depicting a charging state of a battery in the case where the ambient temperature is high, and FIGS. 6A and 6B are graphs for depicting a charging state of a battery in the case where the ambient temperature is low. Referring to FIGS. 5A and 6A, the curves I and I' show the characteristic of the battery voltage (V) with respect to the charging time, and the curves V and V' show the increase (.DELTA.T) of the battery temperature with respect to the charging time. Referring to FIGS. 5B and 6B, the curves VI and VI' show the charging current (Ic) and the curves VII and VII' show the internal pressure (P) of the battery.
Now consider a case where the ambient temperature (Tamb) is relatively high, say 25.degree. C. On the other hand, it is assumed that the thermal switch 5 has been set to be operable at 47.degree. C. In such a case, when the increase (.DELTA.T) of the battery temperature reaches 22.degree. C., the thermal switch 5 is operable and the charging current is controlled. By selecting the increase (Ta) of the battery temperature at charge completion to be 30.degree. C., the battery will not be overcharged in the case where the ambient temperature is high. By contrast, in the case where the ambient temperature is low, say 0.degree. C., the thermal switch 5 will not be operable, unless the increase (.DELTA.T) of the battery temperature reaches 47.degree. C. Assuming that the increase (Ta) of the battery temperature at charge completion of the battery is 30.degree. C., as described above, then it follows that the thermal switch 5 will be operable at the time point which is much later than the time point of charge completion in the case where the ambient temperature is low. Accordingly, overcharging will result in such a case. If the battery is overcharged, the internal pressure (P) becomes abnormally high, as shown by the curve VII' in FIG. 6B and leakage of the electrolyte is caused and in an extreme case the battery is damaged. Accordingly, in order to eliminate a possibility of overcharging by the FIG. 4 apparatus, in the case where the ambient temperature is low, it is necessary to change the operating temperature of the thermal switch 5 in association with the change of the ambient temperature; however, such a thermal switch has not yet been available.
FIG. 7 is a schematic diagram of another example of a conventional charging apparatus of interest to the present invention. The FIG. 7 apparatus is adapted to control a charging current through comparison of the reference voltage and the battery voltage, by changing the reference voltage (Vref) in association with the ambient temperature (Tamb), and such apparatus is disclosed in Japanese Utility Model Laying Open Gazette No. 22730/1978, for example. The FIG. 7 apparatus is structured such that a series connection of a thyristor 6 and a battery 1 is connected to a full-wave rectifying circuit 4. The FIG. 7 apparatus further comprises a voltage comparator, i.e. a differential amplifier 14. The differential amplifier 14 comprises a pair of transistors 15 and 16, wherein a terminal voltage of a capacitor 13 is applied to the base electrode of the transistor 15 and the output of a variable resistor 18 is applied to the base electrode of the transistor 16. The variable resistor 18 is connected in parallel with the diode 17. The diode 17 is provided to be responsive to the ambient temperature. Accordingly, the base electrode of the transistor 16 is supplied with a given reference voltage which is changeable incidental to a change of the ambient temperature and the base electrode of the transistor 15 is supplied with a voltage proportional to the battery voltage of the battery 1.
At the beginning of the charging operation, the battery voltage (V) of the battery 1 is small and accordingly the terminal voltage of the capacitor 13 is also small. Therefore, the collector voltage of the transistor 16 constituting the differential amplifier 14 is also low. Accordingly, the transistor 19 is rendered conductive and the gate electrode of the thyristor 6 is supplied with a given gate voltage, so that the thyristor 6 is turned on. Thereafter, as the charging time lapses, the battery voltage of the battery 1, i.e. the terminal voltage of the capacitor 13 rises. When the voltage at the base electrode of the transistor 15 becomes higher than the voltage at the base electrode of the transistor 16, the voltage at the collector electrode of the transistor 16 becomes high and the transistor becomes only slightly conductive. Accordingly, the gate electrode of the thyristor 6 will not be supplied with a gate voltage and thereafter no charging current flows in the battery 1. At that time, the voltage at the base electrode of the transistor 16, i.e. the reference voltage, changes in association with the ambient temperature by means of the diode 17. As a result, according to the FIG. 7 apparatus, a temperature compensating function depending on the ambient temperature is achieved. However, the FIG. 7 apparatus utilizes a change of a forward voltage drop across the diode 17 as a function of the ambient temperature for the purpose of changing the reference voltage (Vref) in association with the ambient temperature. The characteristic of the forward voltage drop of the diode with respect to the ambient temperature is rather linear. Therefore, the characteristic of the reference voltage (Vref) also becomes a linear one. On the other hand, the characteristic of the voltage at charge completion (Va) of the battery 1 with respect to the ambient temperature is a non-linear one, as shown by the curve III in FIG. 2 and the curve III in FIGS. 8A to 8C. Accordingly, the FIG. 7 apparatus also involves a problem to be described subsequently. More specifically, it can be considered that the reference voltage (Vref) can be set with respect to the voltage at charge completion (Va) in the manner as shown in FIGS. 8A, 8B or 8C. Usually the reference voltage (Vref) is set as shown in FIG. 8B. More specifically, since the characteristic of the voltage at charge completion (Va) is a non-linear one, while the characteristic of the reference voltage (Vref) is a linear one, it is impossible to match the characteristics of the reference voltage (Vref) with the characteristics of the voltage at charge completion (Va) throughout a necessary variation range of the ambient temperature. Accordingly, generally it is adapted such that both characteristics may be matched with each other in a middle temperature region which is normally considered as practical. On the other hand, alternatively it may be adapted such that both characteristics may be consistent in a lower temperature region as shown in FIG. 8A or in a higher temperature region as shown in FIG. 8C. In the case of FIG. 8A, the higher the ambient temperature (Tamb), the more non-conductive the transistor 19 (FIG. 7), and the earlier the timing when the thyristor 6 is turned off (the detection time of charge completion). Conversely, in the case of FIG. 8C, the detection time of charge completion is advanced in the lower temperature region. Accordingly, in either case where the apparatus is set as shown in FIG. 8A or as shown in FIG. 8C, undercharging is inevitable. Furthermore, in the case of FIG. 8B, since the characteristic of the voltage at charge completion (Va) and the characteristic of the reference voltage (Vref) were made consistent in the middle temperature region, this approach is somewhat reasonable in that optimum charging is achieved in the middle temperature region the charging of which is most frequent. However, the case of FIG. 8B involves a problem that complete charging can not be controlled in a lower temperature region or a higher temperature region.
On the other hand, of late there are trends toward utilization of rechargeable batteries as a power source even in motor driven machines consuming a relatively large electric power such as an electric drill, a lawn mower, and the like. Such rechargeable batteries as used in such motor driven machines need be capable of discharging a current larger than that of batteries used in smaller type machines such as electric shavers. Accordingly, such rechargeable batteries as used in such motor driven machines come to involve an increased frequency of charging operation and an increased number of batteries as compared with the batteries in small type machines. Therefore, such rechargeable batteries are often housed in a battery package structured to be separated from a main body of the machine, so that the battery package may be detachable from the main body of the machine. It has also been proposed that such battery package as such may be coupled to a charging apparatus separately provided, so that the batteries in the battery package may be charged independently of the main body of the machine. Considering employing such a battery package, the FIG. 7 apparatus further involves a more serious problem. As described previously, such rechargeable batteries used as a power source of a large sized motor driven machine are caused to be discharged with a higher discharge ratio (a discharge current/a battery current capacity). Usually, it is required that the batteries can be discharged at a high discharge ratio as high as say 3 C to 20 C. However, heat is generated in the batteries due to a discharge of such high discharge ratio and the battery temperature is accordingly increased. In addition, since dissipation of heat from the batteries has been blocked by the battery package, the temperature of the battery once increased is liable not to be decreased. Accordingly, a charge voltage can not be accurately controlled with the FIG. 7 apparatus.
More specifically, immediately after the discharge with a higher discharge ratio, the battery temperature is high as compared with the ambient temperature (Tamb) due to heat generated on the occasion of the discharge of the batteries itself. Accordingly, the voltage at charge completion (Va) of the batteries at that time has also been decreased, as shown in FIGS. 2 and 8A to 8C. However, even in such situation, the FIG. 7 apparatus controls a charging operation such that the voltage at charge completion (Va) may be dependent on the ambient temperature (Tamb) at that time. If the batteries are charged in such situation, it follows that the batteries are overcharged after all. The more the difference between the ambient temperature (Tamb) and the battery temperature, the more serious the above described problem. More specifically, in the case where the battery temperature is high, the difference between the actual voltage at charge completion (Va) and the reference voltage (Vref) as temperature compensated by the ambient temperature becomes large and a probability of causing overcharging is increased. Accordingly, in the case where a charging apparatus as shown in FIG. 7 is used, there was no way but to inhibit the charging, when the batteries are in a high temperature such as immediately after the batteries were discharged with a high discharge ratio.